Recently, electronic equipment has been rapidly developed to be more compact, more lightweight, thinner, and more densely mounted. Especially, printed circuit boards have been commercialized in equipment such as radios, etc. and are presently undergoing expanded use in industrial equipment such as telephones, electronic computers, etc., due to their ability to be mass-produced and their high reliability.
Flexible printed circuit boards were originally used as substitutes for wiring and cables, but since they have flexibility and hence not only can be highly densely mounted in narrow, spaces, but also can withstand repeated bendings, their uses have been expanded as circuits for movable parts of electronic equipment, cables and composite parts imparted with connector functions.
At present, those used in equipment such as cameras, electronic calculators, telephones, printers, etc., typically are formed of a flexible copper-clad board composed of a polyimide or polyester film of about 25 .mu.m in thickness and an electrolytic copper foil or foils of about 35 .mu.m thickness deposited on either or both surfaces thereof and having a circuit pattern formed thereon. In addition, those obtained by providing this circuit pattern with through hole plating and further providing a coating of non-metallic substances (so called "cover lay coating") on both surfaces or the outer layer are also used.
For currently used flexible printed circuit boards, films of a polyimide or polyester resin are generally used as a base material, as described in, for example, U.S. Pat. Nos. 4,377,652 and 3,322,881, Japanese Patent Publication Nos. 29793/72 and 40339/77, and Japanese Patent Application (OPI) Nos. 1123/80 and 73034/76.
However, a coefficient of thermal expansion of the lyester resin is high at 20.degree.-250.degree. C., and thus it lacks through hole connection reliability. Further, since curing is sometimes effected by a steam press at 170.degree. C. during production, there is a tendency, when laminated, that not only the adhesion between the resin layers, but also the flexibility thereof, are lowered.
On the other hand, the polyimide film has an advantage that it can be easily soldered at an ordinary soldering temperature (260.degree. C. or higher), but has a disadvantage that since the surface activity is low, bonding with the metal foil is very difficult. In order to solve this adhesion problem, generally a method is practiced of bonding using an adhesive after treating the film surface by chemical treatment using sodium hydroxide, a chromic acid mixture, aluminum hydroxide, etc. However, even when bonding is conducted by such a method, the board obtained generally does not have satisfactory adhesion as a printed circuit board and is poor in chemical resistance, heat resistance, etc. and thus it has various drawbacks, for example, the copper foil is loosened and comes off by etching treatment, solder flow, etc.
In a method of bonding with a metal foil using a thermosetting adhesive having good heat resistance (e.g., an epoxy resin), it is necessary to overlay the adhesive-applied polyimide film with a metal foil and cure the laminate by heating and compression-treating by a press for about 1-20 hours, and thus there are severe problems with respect to productivity, cost, etc.
Also in the process for the production of printed circuit boards, the conventional process of etching a copper-clad laminate has begun to give way to an additive process of directly drawing a circuit pattern on a resin board by electroless plating by applying the plating technique on plastics and also a substractive process which combines such plating and an etching process. The reason why this additive process has come to be widely employed is that it is based on an effective process of simultaneously forming through holes and pattern plating on a resin laminate which is available in a small range of variety in large quantities, and also for the last several years, the functions of reverse pattern resist inks, etc., have been improved and it has become possible to obtain highly developed fine lines. Recently, a resistless process has also been developed which comprises directly irradiating ultraviolet light on a photo-reactive catalyst incorporated in an adhesive layer through a photomask to deposit metal nuclei, thus effecting electroless plating.
However, with conventional boards composed of paper - phenolic resin, glass - epoxy resin, polyimide resin, etc., used in the above additive process, since the adhesion with the plating is poor, blisters and peel-off can occur, or since the coefficient of thermal expansion in the thickness direction is great, when thermal stress is applied, cracking often takes place, especially in a minute pattern plated part. Further, on making holes in a laminate, it is the present situation that smear (misalignment of holes through the laminate) is generated and interferes with the connection with the plating, and thus the connection reliability is deteriorated. Furthermore, in the etching or plating step, since great quantities of various aqueous solutions and washing water are used, it is an essential requirement that the dimensional change when wet be small.
Furthermore, copper-clad laminates are mainly used as printed circuit boards, and demand therefor has recently been increasing. Heretofore, a phenol resin-impregnated base material and an epoxy resin-impregnated base material have been used for the copper-clad laminates.
However, printed circuit boards employing base materials impregnated with these resins are not always satisfactory in various characteristics; for example, the adhesive power between the copper foil and the base material, the dimensional stability, and the electrical characteristics (especially electrical characteristics after moisture absorption treatment) are greatly lowered, and so on. Therefore, they cannot fully satisfy the demands for densification and higher reliability accompanying recent developments in the electronics fields, and thus printed circuit boards having improved capabilities are anxiously desired.
For improving characteristics such as heat resistance and dimensional stability, fibrous materials have been introduced composed of glass fiber, such as glass paper, filaments and staple fibers, or base materials obtained by impregnating glass fabrics with a phenolic resin or an improved epoxy resin has been used. However, in these circuit boards, although the dimensional stability, heat stability, etc., can be improved to some extent by the presence of these inorganic fibrous materials in the base materials, the moisture resistance, electrical characteristics, etc., cannot be substantially improved. Further, since there is no sufficient adhesive for adhering these base materials and the copper foil, it is the present situation that the improvements in the properties of the base material itself are not sufficiently manifested when the copper foil and the laminate are formed.
Further, where thermoplastic resins are used as base materials, the dimensional stability of the boards obtained is remarkably influenced by the bonding conditions using either heat fusion or the adhesive. In addition, since residual stress of the adhesive layer acts to generate shrinkage at high temperatures, bending, twisting, etc., may be brought about. In order to overcome this problem, those having a glass cloth or mat, an inorganic fiber cloth, etc. incorporated in a base material resin are employed. However, since adhesion and compatibility between the thermoplastic resin and the glass fiber are extremely poor, it is often observed that when repeatedly bent, peel-off occurs between the resin and the glass fiber, thus causing rupture, or break-down of the circuit.
Under such circumstances, there has been a demand for a printed circuit board having not only good dimensional stability at high temperatures, but also excellent electrical characteristics, and still being highly reliable and inexpensive.
In the recent years, with the advance in densification, multiple layering has been developed accordingly. In particular, in a structure of three or more layers, through holes are utilized in order to connect different electrically conductive layers. For example, a plated through hole process, an additive process, a multiwire process, etc., are conducted, and the through holes are utilized also in mounting electronic parts, for example, in a clearance process, a pin inserting process, etc.
In these through holes, it is general to form a conductor by a plating technique, but there are problems in respect to moisture resistance, bond strength, etc. and hence disconnection occurs or the product yield is very poor, and therefore, a resin having good adhesion (i.e., as an insulating layer) has been sought.
Moreover, thermosetting resin laminates generally used as printed circuit boards have not only electrical characteristics but also heat stability, and also, by making the best use of the feature of having only a small dimensional change in the thickness direction, they are used as double-sided through hole printed circuit boards. Heretofore, thermosetting laminates that have been used include laminates obtained by impregnating a base material such as paper, cloth, glass fiber, etc., with a thermosetting resin such as a phenolic resin, an epoxy resin, an unsaturated polyester resin, etc., then drying and thereafter laminating the desired number thereof and finally heating and compression-treating them to mold them together using a thermal press machine. Further, a disadvantage of poor heat dissipating properties is ameliorated by incorporating a good heat conductive powder filler.
When using these thermosetting laminates as insulators for printed circuit boards, it is necessary to laminate an electrically conductive metal foil via an adhesive layer on the laminate. As the electrically conductive metal foil, an electrolytic copper foil or a milled copper foil is used. However, finding an appropriate adhesive for bonding the copper foil and the thermosetting resin is a problem. That is, in general, an epoxy resin is used as the adhesive, but since volatile substances are contained in the adhesive, not only are blisters generated and the conductive layer comes off on heating, but also the dielectric strength is decreased, and so forth. Inter alia, the greatest disadvantages are heat resistance and moisture resistance, and since such properties in the adhesive layers are poorer than those in the thermosetting laminates, it is the present situation that the desirable characteristics of the laminates cannot be fully utilized. Furthermore, since a step of coating the adhesive on the conductor or the laminate is required, the processing steps are very complicated and also there are drawbacks such as that the bonding power is reduced, by, e.g., uneven coating of the adhesive, etc., and that it takes a prolonged time for the adhesive to cure, etc.
Under such circumstances, the development of an adhesive layer which can bond a thermosetting resin laminate and a conductor and has excellent heat resistance, moisture resistance, and insulating properties has been sought.
Some of the present inventors conducted extensive investigations in order to overcome the above-described problems and provided a printed circuit board having good heat resistance and also excellent electrical insulating properties, as described in European Patent Application No. 0148157-A (which corresponds to U.S. Patent Application Serial No. 688,924 filed Jan. 4, 1985), which comprises a laminate of:
(A) a thrn-wall body having a thickness of from 3 .mu.m to less than 5 mm which is a crosslinked product of a mixture comprised of (1) at least one of an ethylene-acrylic acid copolymer and an ethylene-methacrylic acid copolymer (hereinafter collectively referred to as "copolymer (1)") and (2) a saponified product of an ethylene-vinyl acetate copolymer (hereinafter referred to as "copolymer (2)"), wherein the mixing proportion of copolymer (1) in the mixture is from 20 to 80% by weight and the mixture is crosslinked such that after the extraction of the crosslinked product with a boiled toluene for three hours the amount of residue having a size of 0.1 .mu.m or more is at least 80% by weight based on the weight of the crosslinked product; and
(B) an electrically conductive metal layer having a thickness of from 100 .ANG. to 400 .mu.m.
By heating copolymers (1) and (2) at a high temperature a thin-wall body having good heat resistance can be obtained. However, when the thin-wall body is laminated on an electrically conductive metal layer or interposed between the metal layer and a layer of other material(s), to prepare a printed circuit board, uniform adhesive strength is difficult to obtain between the thin-wall body and the metal layer or the layer of other material(s).